All politicians lie.
The difference between Ulfric and Tullius is Ulfric wears his heart on his sleeve for the Nords. Tullius only wants the Nords to stay true to the Empire because it is all about power. They lost a few providences during the Great War and now stand to lose Skyrim. Loosing Skyrim would be a huge blow for the Empire since Nords are very strong and capable warriors.
Tullius plan of action is to lie to the Jarls (you see this in Solitude) and shake hands with the enemy. At the end, Tullius surrenders.
Tullius: I surrender!
Ulfric: The Empire I knew would never surrender.
Tullius: That Empire is dead.
After playing Oblivion and seeing the Empire then and seeing how they are now the Empire has become what they made themselves to be, which is nothing more then weaklings trying to hold on to an Empire that they themselves could not hold on too.
While in the end the Empire will probably over throw the Thalmor (for story reasons to not end the ETS), I think what is best for Skyrim is to detach themselves from a crumbling Empire and no longer be the puppet. And I for one cannot blame those who choose to join the Stormcloaks-not one bit.